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United States Patent |
5,177,225
|
Ramachandran
,   et al.
|
January 5, 1993
|
Process for the production of alkylene oxide
Abstract
Disclosed is a process for the production of an alkylene oxide in which an
alkene, and an oxygen containing gas are reacted in the presence of a
flame suppressor under conditions of low alkene conversion and high
alkylene oxide selectivity in which unreacted alkene is recycled to the
reactor and there is efficient removal of nitrogen and carbon dioxide.
Inventors:
|
Ramachandran; Ramakrishnan (Allendale, NJ);
MacLean; Donald L. (Annandale, NJ)
|
Assignee:
|
The BOC Group, Inc. (Murray Hill, NJ)
|
Appl. No.:
|
549886 |
Filed:
|
July 9, 1990 |
Current U.S. Class: |
549/534; 549/536; 585/821 |
Intern'l Class: |
C07D 301/08 |
Field of Search: |
549/534,536
585/821
|
References Cited
U.S. Patent Documents
Re20370 | May., 1937 | Lefort | 549/534.
|
2125333 | Aug., 1938 | Carter | 549/534.
|
2270780 | Jan., 1942 | Berl | 549/534.
|
3091622 | May., 1963 | Courter et al. | 549/534.
|
3119837 | Jan., 1964 | Kingsley et al. | 549/534.
|
4498910 | Feb., 1985 | Benkmann | 585/821.
|
4769047 | Sep., 1988 | Dye | 585/821.
|
Primary Examiner: Ivy; C. Warren
Assistant Examiner: Trinh; Ba K.
Attorney, Agent or Firm: Reap; Coleman R., Cassett; Larry
Claims
We claim:
1. A process for the production of an alkylene oxide from a corresponding
alkene comprising:
(a) Feeding the alkene, an oxygen containing gas and a flame suppressor to
a reaction zone;
(b) reacting the alkene and oxygen containing gas in said reaction zone
under conditions of low alkene conversion and high selectivity to produce
a mixture of alkylene oxide and off gases;
(c) quenching the mixture obtained in step (b) and removing substantially
all of the alkylene oxide from said quenched mixture;
(d) subjecting all of the alkylene oxide-free off-gases in said quenched
mixture to pressure swing adsorption with an adsorbent which
preferentially adsorbs unreacted alkene, thereby producing a first stream
enriched in unreacted alkene and a second stream enriched in carbon
dioxide; and
(e) forwarding all of said first stream to the reaction zone and removing
said second stream.
2. The process of claim 1 further comprising venting the second stream.
3. The process of claim 1 further comprising incinerating the second
stream.
4. The process of claim 1 further comprising purifying the alkylene oxide
removed from the quenched mixture.
5. The process of claim 1 wherein the oxygen containing gas is air and the
flame suppressor is a combination of nitrogen gas present in the air and
hydrocarbons present in the first stream.
6. The process of claim 1 wherein the oxygen containing gas is selected
from oxygen and oxygen enriched air and the flame suppressor is an alkane
alone or in combination with nitrogen.
7. The process of claim 1 wherein the total amount of flame suppressor is
in the range of about 20 to 80% by volume.
8. The process of claim 1 wherein the alkene is ethylene.
9. The process of claim 1 wherein the alkene is propylene.
10. The process of claim 1 wherein step (c) comprises treating the mixture
obtained in step (b) with water.
11. The process of claim 1 wherein the carbon dioxide adsorbent is a carbon
molecular sieve.
12. A process for the production of an alkylene oxide from a corresponding
alkene comprising:
(a) Feeding the alkene, an oxygen-containing gas and a flame suppressor to
a reaction zone;
(b) reacting the alkene and oxygen containing gas in said reaction zone
under conditions of low alkene conversion and high selectivity to produce
a mixture of alkylene oxide and off gases;
(c) quenching the mixture obtained in step (b) and removing substantially
all of the alkylene oxide from said quenched mixture;
(d) separating the alkene oxide-free off-gases from step (c) into an
alkene-enriched stream, a carbon dioxide-enriched stream and a stream
enriched in the remaining off-gases by passing all of the off-gases from
step (c) through a first pressure swing absorber which contains one of a
carbon dioxide absorbent or a hydrocarbon absorbent, then through a second
pressure swing adsorber which contains the other of a carbon dioxide
absorbent or a hydrocarbon absorbent; and
(e) forwarding all of the alkene-enriched stream to the reaction zone.
13. The process of claim 12 wherein the carbon dioxide adsorbent is
molecular sieve.
14. The process of claim 12 wherein the hydrocarbon adsorbent is selected
from molecular sieve, activated carbon, silica gel and mixtures of these.
15. The process of claim 1 wherein the reaction of the alkene and the
oxygen containing gas is conducted at a temperature of from about
200.degree. to 500.degree. C. and a pressure of from about 15 to 400 psig.
16. The process of claim 1 wherein the off-gases are separated at a
temperature of from about 10.degree. to 100.degree. C. and a pressure of
from about 0 to 400 psig.
Description
TECHNICAL FIELD
The present invention is directed to a process for the production of
alkylene oxides from alkenes and an oxygen-containing gas in which
unreacted alkenes are recovered and recycled to improve the process
efficiency and the off-gases are treated without incineration which saves
natural resources and provides for the accumulation of carbon dioxide as a
by-product.
BACKGROUND OF THE PRIOR ART
The production of alkylene oxides from alkenes in the presence of suitable
catalysts is well known. Brian J. Ozero, Handbook of Chemicals Production
Processes, edited by Robert Meyers, McGraw Hill Book Co. (1986) at Chapter
1.5, discloses cyclic processes using both oxygen and air as an oxidant
for the production ethylene oxide from ethylene. In these processes, the
alkene is oxidized in a multitubular catalytic reactor in vapor phase. The
reactor off gases are cooled and scrubbed with water in an absorber to
recover ethylene oxide which is sent to a recovery section for further
purification.
In the oxygen-based process described by Ozero, the scrubber off gases are
divided into three parts which are respectively: i) recycled to the
reactor, ii) vented and iii) sent to a separator for carbon dioxide
removal and recycle of the remaining hydrocarbons. This process suffers
from several disadvantages. In particular, the process requires a separate
carbon dioxide removal unit and a purge to remove argon which would
otherwise accumulate in the system.
In the air-based process described by Ozero, the scrubber off gases are
sent to a second reactor, which is the purge reactor, where additional
unreacted ethylene is reacted using a higher air to ethylene ratio,
thereby foregoing some ethylene oxide selectivity. The reactor off gases
are passed through another water scrubber to recover ethylene oxide.
It is known that the volume of hydrocarbons purged, when utilizing air as a
source of oxygen, requires that the purge scrubber off gases be
incinerated to remove any remaining hydrocarbons in order to meet
environmental regulations. In this air-based process, an additional purge
oxidation reactor, a water scrubber, and an effluent incinerator are
required, as well as a greater volume of catalyst. Another shortcoming of
the processes described by Ozero is that they are for practical purposes
limited to the use of either oxygen or air. It would be advantageous to
eliminate the purge and additional carbon dioxide separator and operate
the ethylene oxide reactor at a higher selectivity to improve the overall
process efficiency.
SUMMARY OF THE INVENTION
The present invention is directed generally to a process for the production
of alkylene oxides by the reaction of an alkene and an oxygen-containing
gas. The process of the present invention employs a separation system in
which substantially all of the unreacted alkene is removed from the
scrubber off gases and recycled back to the reactor. This enables the
reaction to be conducted at low conversion, high selectivity while the
separation system off gases may be vented directly. Specifically, the
present invention is directed to a process for the production of an
alkylene oxide from a corresponding alkene which comprises feeding the
alkene, an oxygen containing gas and a flame suppressor to a reaction
zone. This flame suppressor can be fed either continuously or only during
start-up. The gases are reacted under conditions of low alkene conversion
and high alkylene oxide selectivity to produce a mixture of alkylene oxide
and off gases.
As used herein, the term "low conversion" and "high selectivity" means
rates of conversion and selectivity which are respectively lower and
higher than processes typically practiced in the prior art. More
specifically, the term "low conversion" is a rate of conversion which
results in an increase in the rate of selectivity of at least 1% compared
to conventional once-through process which do not recycle the alkene to
the oxidation reactor. By way of example only, the rates typically
employed in the present invention for the conversion of alkene to alkylene
oxide are in the range of from about 5 to 80% most typically from about 10
to 60%. Corresponding selectivity rates are in the range from about 50 to
90% for ethylene oxide and 15 to 80% for propylene oxide.
The mixture is quenched to remove the alkylene oxide produced for further
purification in a manner known to those skilled in the art.
The remaining off gases are sent to at least one separator, either with or
without compression, to produce a first stream containing unreacted alkene
and the flame suppressor and a second stream containing the remaining
gases. The first stream is recyled back to the reactor to be combined with
fresh oxygen-containing gas and fed into the reactor. The second stream is
removed and may be vented, incinerated or further separated to remove
purified carbon dioxide.
In accordance with the present invention oxygen-enriched air can be used as
the oxidant because the system provides for the effective removal of
nitrogen gas while taking advantage of the positive effects of conducting
the process under high alkylene oxide selectivity at low alkene conversion
rates.
In addition, the efficient removal of nitrogen and carbon dioxide from the
system reduces the outlay for capital equipment by making the flow rates
lower and thereby eliminating the need for a separate carbon dioxide
removal system.
Further, when oxygen enriched air is used as the oxidant in the present
invention, it is not necessary to use methane to render the reactants
inflammable as customarily employed in prior art systems based on oxygen.
This is because the nitrogen present in the air and the recycled carbon
dioxide provide sufficient flame suppression without undesirable nitrogen
gas build-up.
BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings in which like reference characters indicate like
parts are illustrative of embodiments of the invention and are not
intended to limit the invention as encompassed by the claims forming part
of the application. In particular, the embodiments are described in
connection with the production of ethylene oxide from ethylene. It should
be understood, however, that such embodiments are applicable to the
production of propylene oxide from propylene and the like.
FIG. 1 is a schematic view of a prior art system for converting ethylene to
ethylene oxide using air as the oxidant;
FIG. 2 is a schematic view of a prior art system for converting ethylene to
ethylene oxide using pure oxygen as the oxidant;
FIG. 3 is a schematic view of an embodiment of the present invention for
converting an alkene to the corresponding alkylene oxide using pure oxygen
or other oxygen-containing gas as the oxidant; and
FIG. 4 is a schematic view of another embodiment of the present invention
similar to the embodiment shown in FIG. 3 in which multiple adsorbers are
used for separation.
DETAILED DESCRIPTION OF THE INVENTION
Prior processes for the production of ethylene oxide by the oxidation of
ethylene have employed either air or pure oxygen as the oxidant. FIG. 1
shows a prior art system using air as the oxidant and a purge to remove
the inert gases to prevent nitrogen build up and an incinerator to meet
environmental regulations.
Specifically air and ethylene are fed to an oxidation reactor 2 containing
a catalyst composed, for example, of a metal on a support such as silver
on alumina. A mixture of ethylene oxide and off gases are cooled and then
fed to a scrubber 4 in which water is used to dissolve the ethylene oxide
for subsequent treatment. The off gases including unreacted ethylene are
removed and divided into two streams. A first stream is returned via a
line 6 to the oxidation reactor 2 while a second stream is sent via a line
8 to a purge reactor 10.
The second stream containing off gases including oxygen, nitrogen, argon,
ethylene and carbon dioxide is combined with additional quantities of air
in the purge reactor 10 to provide a relatively high oxygen to ethylene
ratio which obtains a higher conversion of ethylene and thereby produces
additional quantities of ethylene oxide. The by-products of the reaction
are sent to a second scrubber 12 in which ethylene oxide is recovered and
a stream of off gases containing unreacted ethylene is divided into two
streams.
A first stream is fed via a line 14 back to the purge reactor 10 and the
second stream is sent to an incinerator 16, where the off-gases,
particularly the hydrocarbons contained therein, are combusted and
thereafter vented.
Referring to FIG. 2, there is shown a prior art system in which pure oxygen
gas is used as the oxidant. Ethylene, oxygen and a flame suppressor such
as methane gas are sent to the oxidation reactor 2 of the same type
described in connection with FIG. 1. Ethylene oxide and off gases are sent
to the scrubber 4 for recovery of ethylene oxide for purification. The
off-gases are divided into three streams, one stream flows via the line 6
back to the oxidation reactor 2. A second stream is sent to an incinerator
for combusting the hydrocarbons and a third stream is sent to an absorber
18 for removing carbon dioxide from the off gases. A portion of the off
gases removed from the absorber 18 is sent to the oxidation reactor 2 and
the remaining off gases are recycled to the absorber 18.
In accordance with the present invention, there is provided a system for
the conversion of an alkene to an alkylene oxide in which the oxidant can
be selected from any one or more of pure oxygen, air and oxygen enriched
air. The oxidants can be combined and the composition of the oxidants can
be changed without the need for material changes in capital equipment
depending on the alkylene oxide requirement. As a consequence, the system
of the present invention provides greater flexibility in the use of
oxidants over known systems.
Referring to FIG. 3, the process of the present invention commences by
forwarding a gaseous alkene via a line 50 and an oxygen containing gas via
a line 52 to an oxidation reactor 54. The starting alkenes have from 2 to
4 carbon atoms, particularly ethylene and propylene.
The oxidation reactor contains a suitable oxidation catalyst, such as
silver on alumina, in a fixed, fluidized or slurry reactor. The catalyst
may be promoted with other known metals to improve stability and
selectivity.
As previously indicated, the oxidant may range from pure oxygen to air. The
optimum oxygen concentration will depend on whether the process is used to
retrofit an existing plant or implemented as a new plant or, if
retrofitted, the need for additional capacity. In other words, the process
can be employed without major modifications in plants of varying capacity.
It is also necessary in accordance with the present invention to provide
for a flame suppressor. In the case of pure oxygen, methane or ethane may
be used as a flame suppressor. The amount of the flame suppressor is
controlled so as to avoid the formation of flammable mixtures in the
system. Typically, the total amount of flame suppressors is in the range
of about 20 to 80%. A major portion of the flame suppressor is added only
during the start up since most of the flame suppressor is recycled. For
the process using air as the oxygen containing gas, typical nitrogen
concentrations are about 30% by volume and carbon dioxide concentrations
about 20% as the reactor feed.
The oxidation reaction is conducted at a temperature in the range of from
about 200.degree. to 500.degree. C. and a pressure of from about 15 to 400
psig.
The resulting product mixture includes the alkylene oxide (e.g. ethylene
oxide) unreacted alkene (e.g. ethylene), oxygen and carbon dioxide,
nitrogen, and argon if other than pure oxygen is used as the oxidant.
The mixture is cooled in a cooler (not shown) and then sent via the line 56
to a scrubber 58 wherein water from a line 60 is used to separate the
alkylene oxide from the off gases. The alkylene oxide is removed from the
scrubber 58 through a line 62.
The off gases are sent via a line 64 to a pressure swing adsorber 66
containing two or more beds, preferably in parallel with suitable
adsorbents capable of removing carbon dioxide, nitrogen and argon, if
present in the reactor feed. Typical adsorbents include activated carbon,
silica gel and molecular sieves and other adsorbents well known to those
skilled in the art. The scrubber off gases enter the adsorber 66 at a
temperature of from about 10.degree. to 100.degree. C. and a pressure of
from about 0 to 400 psig. Depending upon the reactor pressure, it may be
necessary to compress the scrubber off gases before feeding it to the
pressure swing adsorber 66.
A first stream exits the adsorber 66 via a line 68 for return to the
oxidation reactor 54. The first stream contains substantially all of the
unreacted alkylene and minor amounts of carbon dioxide, nitrogen, oxygen
and argon.
Because substantially all of the hydrocarbons (e.g. ethylene) leaving the
adsorber 66 are returned in the recycle, a second stream containing off
gases excluding hydrocarbons can be vented via the line 70 without
incineration. As a consequence, the process of the present invention can
operate without the costly incineration apparatus associated with prior
art processes.
Carbon dioxide can be removed as part of the vent gases or separated from
the vent gases and removed as a by-product via a line 72 depending upon
the oxygen concentration in the reactor feed. If air is used as the oxygen
containing gas, carbon dioxide may not be recovered as a by-product. The
separation of carbon dioxide from the vent gases requires a carbon dioxide
adsorbing material such as molecular sieve. Carbon dioxide is removed by
adsorbing it on the molecular sieve preferentially over the remaining
gases and is obtained as a desorbed product.
When air is used as the oxygen containing gas or when very large quantities
of carbon dioxide must be removed, it may be desirable to use more than
one pressure swing adsorption system in series. The first system
preferably is capable of selectively adsorbing carbon dioxide while the
second preferentially adsorbs hydrocarbons.
Referring to FIG. 4, there is shown the use of two pressure swing
adsorption columns. The off gases from the scrubber 58 are sent via the
line 64 to a first pressure swing adsorber 80 containing adsorbents which
preferentially adsorb carbon dioxide as described previously. Carbon
dioxide is removed as a by-product via a line 82. The remaining off gases
are sent via a line 84 to a second pressure swing adsorber 86 containing
adsorbents which preferentially remove hydrocarbons (e.g. ethylene) from
the off gas stream via a line 88 for recycling. Nitrogen, oxygen and other
off gases can be vented from the second pressure swing adsorber 86 via a
line 90 without incineration.
EXAMPLE 1
The process of the present invention was conducted in accordance with FIG.
3 in the following manner to produce ethylene oxide. 141 moles of
ethylene, and 1203.5 moles of air (containing 252.6 moles of oxygen,
950.2 moles of nitrogen and a trace amount of ethylene) were forwarded via
the lines 50, 52 respectively into the oxidation reactor 54. In addition,
the reactor 54 was supplied with a recycle of ethylene and other off gases
via the line 68 to raise the quantity of the gases therein to that shown
in Table 1.
TABLE 1
______________________________________
CONTENTS TO THE REACTOR
Gas Moles % by volume
______________________________________
ethylene 1293.7 35.7
ethane 72.4 2.0
oxygen 281.2 7.8
carbon dioxide 792.9 21.9
nitrogen 1187.8 32.7
______________________________________
The gas mixture set forth in Table 1 produces the gas mixture shown in
Table 3 as the product. This product was forwarded via the line 56 to the
scrubber 58.
TABLE 2
______________________________________
CONTENTS TO THE SCRUBBER
Gas Moles % by volume
______________________________________
ethylene 1164.3 32.5
ethane 72.4 2.0
oxygen 143.1 4.0
ethylene oxide 100.0 2.8
carbon dioxide 851.6 23.8
water vapor 58.7 1.6
nitrogen 1187.8 33.2
______________________________________
100 moles of ethylene oxide were removed from the scrubber 58 via the line
62 to provide a conversion rate of ethylene to ethylene oxide of 10.0% and
a selectivity of 77%.
After quenching, the gases were sent to a pressure swing adsorber 66 to
separate ethylene and, optionally carbon dioxide from the off gases. The
charge sent to the pressure swing adsorber had the composition shown below
in Table 3.
TABLE 3
______________________________________
CONTENTS TO THE PSA
Gas Moles % by volume
______________________________________
ethylene 1164.3 33.8
ethane 72.4 2.1
oxygen 43.1 4.2
carbon dioxide 881.0 25.5
nitrogen 1187.8 34.4
______________________________________
The temperature in the pressure swing adsorber 66 was in the range from
about 15.degree. to 35.degree. C. and a pressure of from about 5 to 100
psig.
Substantially all of the ethylene (1152.7 moles; 99+%) was sent via the
line 68 to the oxidation reactor 54. A gas mixture containing 11.6 moles
of ethylene, 0.7 moles of ethane, 74.9 moles of oxygen and 88.1 moles of
carbon dioxide was vented out of the system via the line 70. Depending
upon the hydrocarbon recovery in the pressure swing adsorber, it may be
necessary to incinerate the vent stream.
EXAMPLE 2
The process of the present invention was conducted in accordance with FIG.
3 using pure oxygen as the oxidant to produce ethylene oxide. 141.0 moles
of ethylene, 213.0 moles of oxygen and a trace amount of ethane was
forwarded via the lines 50 and 52, respectively to the oxidation reactor
54. In addition, the reactor 54 was supplied with a recycle of ethylene
and other gases via the line 68 to raise the quantity of gases therein to
that shown in Table 4.
TABLE 4
______________________________________
CONTENTS TO THE REACTOR
Gas Moles % by volume
______________________________________
ethylene 1293.7 54.1
ethane 72.4 3.0
oxygen 231.7 9.7
carbon dioxide 792.9 33.2
______________________________________
The gas mixture shown in Table 4 was reacted to produce the stream shown in
Table 5 which was forwarded via the line 56 to the scrubber 58.
TABLE 5
______________________________________
CONTENTS TO THE SCRUBBER
Gas Moles % by volume
______________________________________
ethylene 1164.3 49.7
ethane 72.4 3.1
oxygen 93.6 4.0
ethylene oxide 100.0 4.3
carbon dioxide 851.6 36.4
______________________________________
The ethylene oxide was removed from the scrubber 58 to provide a conversion
rate of ethylene to ethylene oxide of 10.0% and a selectivity of 77%.
After quenching the gases were sent to a pressure swing absorber 66 to
separate ethylene and, optionally carbon dioxide from the off gases. The
charge sent to the pressure swing adsorber had the composition shown below
in Table 6.
TABLE 6
______________________________________
CONTENTS TO THE PSA
Gas Moles % by volume
______________________________________
ethylene 1164.3 52.7
ethane 72.4 3.3
oxygen 93.6 4.2
carbon dioxide 881.0 39.8
______________________________________
The temperature in the adsorber 66 was the range from about 15.degree. to
35.degree. C. and a pressure of from about 5 to 100 psig.
Substantially all of the ethylene (1152.7 moles; 99+% by volume) was sent
via the line 68 to the oxidation reactor 54. A gas mixture containing 11.6
moles of ethylene, 0.7 moles of ethane and 74.9 moles of oxygen and 88.1
moles of carbon dioxide was vented out of the system via the line 70.
Depending on the amount of hydrocarbon in the stream, it may be necessary
to incinerate the vent stream.
EXAMPLE 3
The process of the present invention was conducted in accordance with FIG.
3 in the following manner to produce propylene oxide. 405.1 moles of
propylene, 8.3 moles of propane and 558.4 moles of oxygen, were forwarded
via the lines 50, 52 respectively into the oxidation reactor 54. In
addition, the reactor 54 was supplied with a recycle of propylene and
other off gases via the line 68 to raise the quantity of the gases therein
to that shown in Table 7.
TABLE 7
______________________________________
CONTENTS TO THE REACTOR
Gas Moles % by volume
______________________________________
propylene 2461.5 58.5
propane 698.9 16.6
oxygen 625.7 14.9
carbon dioxide 336.2 8.0
ethylene 84.9 2.0
formaldehyde 1.9 0.0
______________________________________
The gas mixture set forth in Table 7 produces the gas mixture shown in
Table 8 as the product. This product was forwarded via the line 56 to the
scrubber 58.
TABLE 8
______________________________________
CONTENTS TO THE SCRUBBER
Gas Moles % by volume
______________________________________
propylene 2082.5 49.7
propane 698.9 16.7
oxygen 83.8 2.0
propylene oxide 100.0 2.4
acetaldehyde 134.6 3.2
formaldehyde 64.9 1.5
carbon dioxide 594.8 14.2
ethylene 153.8 3.7
water vapor 241.0 5.7
Balance (alcohol,
38.0 0.9
acetone, dienes)
______________________________________
100 moles of propylene oxide were removed from the scrubber 58 via the line
62 to provide a conversion rate of propylene to propylene oxide of 15.4%
and a selectivity of 26.4%.
After quenching, the gases were sent to a pressure swing adsorber 66 to
separate propylene and, optionally carbon dioxide from the off gases. The
charge sent to the pressure swing adsorber had the composition shown below
in Table 9.
TABLE 9
______________________________________
CONTENTS TO THE PSA
Gas Moles % by volume
______________________________________
propylene 1300.6 52.0
propane 413.8 16.5
oxygen 82.8 3.3
carbon dioxide 560.3 22.4
ethylene 141.5 5.7
formaldehyde 3.2 0.1
______________________________________
The temperature in the pressure swing adsorber 66 was in the range from
about 15.degree. to 35.degree. C. and a pressure of from about 5 to 100
psig.
1274.6 moles of propylene was sent via the line 68 to the oxidation reactor
54. A gas mixture containing 26.0 moles of propylene, 8.3 moles of
propane, 16.6 moles of oxygen, 224.1 moles of carbon dioxide and 56.6
moles of ethylene was vented out of the system via the line 70. Depending
upon the hydrocarbon recovery in the pressure swing adsorber, it may be
necessary to incinerate the vent stream.
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